Lithium ion batteries, having such characteristics as large energy density and a long life span, are used widely as power sources for home appliances such as video cameras and portable electronic devices such as notebook computers and mobile phones, electric tools such as power tools, and the like, and recently have been put into application in large batteries that are installed in an electric vehicle (EV), a hybrid electric vehicle (HEV) and the like.
A lithium ion battery of such type including a positive electrode, a negative electrode, and an ion conducting layer which is placed between two electrodes. As an ion conducting layer, a separator including a porous film like polyethylene and polypropylene, which is filled with non-aqueous electrolyte liquid, is used.
A separator is a member for separating a positive electrode from a negative electrode in a battery, and by maintaining electrolyte liquid therein, it also plays a role of ensuring ionic conductivity between them. As a separator for a lithium ion battery, an electrochemically inactive porous body (including a porous membrane), that can form a passage for lithium ion conductivity between electrodes by maintaining electrolyte liquid in voids, is used, for example. In general, a microporous polyolefin film containing polyethylene or polypropylene is used.
As a negative electrode active material for constituting a negative electrode, a material capable of intercalating reversibly lithium in ionic state is used. As a negative electrode active material, a carbon material, a silicon oxide series compound like silicone and Si—O, lithium titanate, tin alloy, or a mixture containing such negative electrode material as a main component is generally used, and graphitic carbon materials, pitch cokes, fibrous carbon, or the like are known as an example.
A lithium ion battery is a secondary battery having a structure in which, during charging, lithium is eluted from the positive electrode as ions and moves toward the negative electrode to be stored and conversely, during discharging, the lithium ion returns from the negative electrode to the positive electrode, and it is known that the high energy density of the battery is based on the electric potential of the positive electrode material.
In addition to lithium metal oxides such as LiCoO2 and LiMnO2 having a layer structure, lithium metal oxides having a spinel structure (Fd3-m) of the manganese series such as LiMn2O4 and LiNi0.5Mn0.5O4 are known as a positive electrode active material used for a lithium ion battery. Among them, since the spinel type lithium metal oxides (herein, it is also referred to as “LMO”) of the manganese series have low raw material costs, are free of toxicity, which renders it safe, and furthermore, have properties being strong against overcharging, there is a focus on the spinel type lithium transition metal oxide (LMO) of the manganese series as a next-generation positive electrode active material for a large battery for an electric vehicle (EV), a hybrid electric vehicle (HEV) and the like. In addition, an LMO, which allows three-dimensional insertion and desorption of Li ions, has excellent output characteristics, compared to a lithium transition metal oxide such as LiCoO2, which has a layer structure, such that utilization in applications requiring excellent output characteristics such as tools called power tools, EV and HEV batteries and the like, are anticipated, and thus additional improvement of output characteristics are waited for.
However, there has been a problem about a lithium ion battery that, according to increasing number of charge and discharge, a dendrite grows near a negative electrode during storage at high temperatures, which causes short circuit between electrodes. It is believed that dendrite growth is caused by many reasons, and foreign materials like iron included in a positive electrode active material is considered to be one reason. Specifically, it is believed that foreign materials like iron or stainless steel that is incorporated in a positive electrode material are eluted, precipitated on a negative electrode or the like, and generate a dendrite consequently.
Under the circumstances, to prevent incorporation of foreign materials like iron into a battery material, a method of removing iron by passing a lithium compound or a transition metal compound through magnetic field with pre-determined strength before calcining a mixture containing the lithium compound and transition metal compound is suggested (see Patent Document 1, JP 2003419026 A; Patent Document 2, JP 2003-119029 A; and Patent Document 3, JP 2003-313520 A).